There is much literature and many patents relating to the reduction of silver images, particularly halftone or line silver images formed on plate making photographic materials, and various reducers have been conventionally used.
Farmer's reducer mainly (composed of red prussiate) had generally been used in the past. Cerium sulfate reducer and ethylenediaminetetraacetic acid ferric chelate reducer have been used in recent years from the viewpoint of preventing environmental pollution.
As plate making photographic materials, lith films have been mainly used. New high-contrast materials containing hydrazine compounds or tetrazolium compounds have recently been used and the required characteristics of the reducers used have been changed.
Farmer's reducer is neutral and has good reduction characteristics, but has disadvantages in that it causes environmental pollution and it is inferior in shelf stability.
Cerium sulfate reducer has good shelf stability and is a relatively good reducer. However, it has disadvantages in that it is highly acidic, hard to handle and yellow staining is liable to be caused in non-image areas.
Ethylenediaminetetraacetic acid ferric chelate reducer is neutral and inexpensive, but is poor in reduction ability. When novel high-contrast photographic materials having a high silver iodide content are used, a yellowish brown residue is formed around silver images and, hence, the effective reduction degree is decreased. Further, there is the problem that the image area after reduction has a yellow stain.
The term "good reducer" means a reducer where the decrease in area of the silver images relative to the loss in density of the silver images is large, i.e., a reducer where the decrease in size is large.
JP-A-49-33701 ("JP-A" as used herein means an "unexamined published Japanese patent application") discloses reducers where amino polycarboxylic acid ferric chelates are used as oxidizing agents. The earlier discussed problems cannot be solved by the amino polycarboxylic acids described therein when they are used alone. Since the amino polycarboxylic acid ferric chelates are generally low in oxidizing power, the reduction (oxidizing) rate is slow as compared with cerium sulfate and red prussiate. Hence, it is difficult to make a reducer which can be practically used. Further, there are the problems: (1) in particular, when used in combination with novel high-contrast photographic materials, the lowering in the density of halftone dots is high , even when reduction is conducted over a long period of time, (2) the image area after reduction has yellow stain; and (3) a yellowish-brown residue is liable to be left around halftone dots.
JP-A-52-68419 discloses that heterocyclic mercapto compounds used alone and in combination with ethylenediaminetetraacetic acid ferric chelate can be used as reducers. However, the earlier described problems are not solved by such materials.